Biomedical subjects

J Tommassen

Publications and source records attributed to J Tommassen.

At least 55 records · Page 3Linked to original sources

PubMed1997

Folding of a bacterial outer membrane protein during passage through the periplasm.

The transport of bacterial outer membrane proteins to their destination might be either a one-step process via the contact zones between the inner and outer membrane or a two-step process, implicating a periplasmic intermediate that inserts into the membrane. Furthermore, folding might precede insertion or vice versa. To address these questions, we have made use of the known 3D-structure of the trimeric porin PhoE of Escherichia coli to engineer intramolecular disulfide bridges into this protein at positions that are not exposed to the periplasm once the protein is correctly assembled. The mutations did not interfere with the biogenesis of the protein, and disulfide bond formation appeared to be dependent on the periplasmic enzyme DsbA, which catalyzes disulfide bond formation in the periplasm. This proves that the protein passes through the periplasm on its way to the outer membrane. Furthermore, since the disulfide bonds create elements of tertiary structure within the mutant proteins, it appears that these proteins are at least partially folded before they insert into the outer membrane.

Blotting, Western↗

PubMed1997

Voltage sensing in the PhoE and OmpF outer membrane porins of Escherichia coli: role of charged residues.

The porins PhoE and OmpF form anion and cation-selective pores, respectively, in the outer membrane of Escherichia coli. Each monomer of these trimeric proteins consists of a 16-stranded beta-barrel, which contains a constriction at half the height of the channel. The functional significance of a transverse electrical field that is formed by charged amino acid residues within the constriction zone was investigated. For this purpose, the PhoE residues R37, R75, K18 and E110 were substituted by neutral amino acids. The mutant pores allowed an increased permeation of beta-lactam antibiotics across the outer membrane in vivo, although the single channel conductance, measured in planar lipid bilayers, was not increased or even slightly decreased. Replacement of the positively charged residues resulted in a decreased voltage sensitivity, whereas the substitution of a negatively charged residue resulted in an increased voltage sensitivity. Similar substitutions in OmpF caused the opposite effects, i.e. the substitution of positive and negative charges resulted in increased and decreased voltage sensitivity, respectively. Together, the results suggest that opposite charges, i.e. positive charges in anion-selective and negative charges in cation-selective porins, act as sensors for voltage gating.

Bacterial Outer Membrane Proteins↗

PubMed1997

Role of the carboxy-terminal phenylalanine in the biogenesis of outer membrane protein PhoE of Escherichia coli K-12.

Most bacterial outer membrane proteins contain a phenylalanine at their C terminus. It has been shown that this residue has an important role in the efficient and correct assembly of PhoE protein into the Escherichia coli outer membrane, since its substitution or deletion resulted in the accumulation of trypsin-sensitive monomers of this normally trimeric protein. Here, the role of the C-terminal Phe in the assembly of PhoE was studied in further detail. Immunocytochemical labelling on ultrathin cryosections revealed that a mutant PhoE protein that lacks the C-terminal Phe accumulates in the periplasm. However, when the expression levels of the altered species were reduced, the efficiency of outer membrane incorporation was increased and the lethal effects were alleviated. The role of the C-terminal Phe in protein folding, trimerization and outer membrane incorporation was further studied in vitro. Deletion of this residue interfered with the efficiency of the formation of an assembly-competent folded monomer, and the stability of this PhoE form was affected. The in vitro trimerization and insertion into outer membranes were not affected by the mutation.

Bacterial Proteins↗

PubMed1997

Dimerization regulates the enzymatic activity of Escherichia coli outer membrane phospholipase A.

The outer membrane phospholipase A (OMPLA) of Escherichia coli is present in a dormant state in the cell envelope. The enzyme is activated by various processes, which have in common that they perturb the outer membrane. Kinetic experiments, chemical cross-linking, and analytical ultracentrifugation were carried out with purified, detergent-solubilized OMPLA to understand the underlying mechanism that results in activation. Under conditions in which the enzyme displayed full activity, OMPLA was dimeric. High detergent concentrations or very dilute protein concentrations resulted in low specific activity of the enzyme, and under those conditions the enzyme was monomeric. The cofactor Ca2+ was required for dimerization. Covalent modification of the active site serine with hexadecylsulfonylfluoride resulted in stabilization of the dimeric form and a loss of the absolute calcium requirement for dimerization. The results of these experiments provide evidence for dimerization as the molecular mechanism by which the enzymatic activity of OMPLA is regulated. This dimerization probably plays a role in vivo as well. Data from chemical cross-linking on whole cells indicate that OMPLA is present in the outer membrane as a monomer and that activation of the enzyme induces dimerization concurrent with the appearance of enzymatic activity.

Detergents↗

PubMed1997

Response of Neisseria meningitidis to iron limitation.

In the human body, the concentration of free iron is limiting for bacterial growth, since iron is bound to transport and storage proteins such as transferrin and lactoferrin. When grown under iron starvation, Neisseria meningitidis produces receptors for these proteins in the outer membrane. These receptors are presently being characterized at the molecular level. Here, we summarize our current knowledge of these receptors, with special emphasis on the LbpA and FrpB proteins, which are studied in our laboratories. Furthermore, the genetic and antigenic variability of these proteins and their vaccine potential are discussed.

Amino Acid Sequence↗

PubMed1997

Two-dimensional structure of the Opc invasin from Neisseria meningitidis.

A two-dimensional structural model was devised for the Opc outer membrane protein invasin which contains 10 transmembrane strands and five surface-exposed loops. One continuous epitope recognized by three monoclonal antibodies was localized to the tip of loop 2 by synthetic peptides and site-directed mutagenesis while a second, discontinuous epitope recognized by a fourth antibody was localized to loops 4 and 5 by insertion mutagenesis. These monoclonal antibodies are bactericidal and inhibit adhesion and invasion. Most of the T-cell epitopes defined by Wiertz et al. (1996) were localized to the transmembrane strands. Oligonucleotides encoding a foreign epitope (nabla) from Semliki Forest virus were inserted into BglII restriction sites created by site-directed mutagenesis. The nabla epitopes inserted in all five predicted loops were recognized on the cell surface of live Escherichia coli bacteria by a monoclonal antibody and are exposed while nabla epitopes in the N-terminus or three predicted turns were not. The results thus confirm important predictions of the model and define five permissive sites within surface-exposed loops which can be used to insert foreign epitopes.

Adhesins, Bacterial↗

PubMed1997

The outer membrane component, YscC, of the Yop secretion machinery of Yersinia enterocolitica forms a ring-shaped multimeric complex.

The YscC protein of Yersinia enterocolitica is essential for the secretion of anti-host factors, called Yops, into the extracellular environment. It belongs to a family of outer membrane proteins, collectively designated secretins, that participate in a variety of transport processes. YscC has been shown to exist as a stable oligomeric complex in the outer membrane. The production of the YscC complex is regulated by temperature and is reduced in strains carrying mutations in the yscN-U operon or in the virG gene. The VirG lipoprotein was shown to be required for efficient targeting of the complex to the outer membrane. Electron microscopy revealed that purified YscC complexes form ring-shaped structures of approximately 20 nm with an apparent central pore. Because of the architecture of the multimer, YscC appears to represent a novel type of channel-forming proteins in the bacterial outer membrane.

Bacterial Outer Membrane Proteins↗

PubMed1997

Pore functioning of outer membrane protein PhoE of Escherichia coli: mutagenesis of the constriction loop L3.

Each monomer of the trimeric outer membrane porin PhoE of Escherichia coli consists of a 16-stranded beta-barrel with short turns at the periplasmic side and large loops at the cell surface. One of these loops, L3, is folded inside the beta-barrel and forms a constriction within the channel. Therefore, it is assumed to play an important role in the permeability properties of this general diffusion pore. Several site-directed mutations were introduced in loop L3 to investigate its function. The loop L3 contains a short alpha-helix and, at the tip of the loop, a highly conserved PEFGG sequence. The alpha-helix was deleted and the two glycines in the PEFGG sequence were either replaced by alanines or deleted. A serine residue, supposed to play an indirect role in the anion selectivity of the pore, was removed. The mutant porins were analysed both in vitro and in vivo. The results suggest that flexibility of the third loop is important for solute passage and that this flexibility is determined by the two glycine residues in the PEFGG sequence. Furthermore, the alpha-helix is probably important for the folding of the protein. The supposed involvement of Ser115 (Ser121A in OmpF nomenclature) in anion selectivity was confirmed.

Anti-Bacterial Agents↗

PubMed1997

Topology of the outer membrane phospholipase A of Salmonella typhimurium.

The outer membrane phospholipase A (OMPLA) of Enterobacteriaceae has been proposed to span the membrane 14 times as antiparallel amphipathic beta-strands, thereby exposing seven loops to the cell surface. We have employed the epitope insertion method to probe the topology of OMPLA of Salmonella typhimurium. First, missense mutations were introduced at various positions in the pldA gene, encoding OMPLA, to create unique BamHI sites. These BamHI sites were subsequently used to insert linkers, encoding a 16-amino-acid B-cell epitope. Proper assembly of all mutant proteins was revealed by their heat modifiability in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The accessibility of the inserted epitopes was assessed. Immunofluorescence analysis of intact cells with antibodies against the inserted epitope showed that three of seven predicted loops are indeed cell surface exposed. Trypsin accessibility experiments verified the cell surface exposure of two additional loops and provided support for the proposed periplasmic localization of three predicted turns. For two other predicted exposed loops, the results were not conclusive. These results support to a large extent the proposed topology model of OMPLA. Furthermore, the observation that the substitutions Glu66Pro and Glu247Gly virtually abolished enzymatic activity indicates that these residues might play a major role in catalysis.

Amino Acid Sequence↗

PubMed1996

Structural and functional characterization of a His-tagged PhoE pore protein of Escherichia coli.

The recent elucidation of the 3-D structure of the outer membrane protein PhoE of Escherichia coli provides an excellent tool for a detailed analysis of the structure-function relationship of this pore-forming protein. For this purpose, a fast and efficient method for the purification of mutant porins is needed. A histidine-tag was engineered between the signal sequence and the N terminus of mature PhoE. The recombinant PhoE protein was normally assembled into the outer membrane and could be purified by immobilized metal affinity chromatography. Part of the total amount of the trimers dissociated into folded monomers during purification. The histidine-tag did not change the electrophysical characteristics of the protein in lipid bilayers. Hence, the method is useful for the fast purification of mutant porins for functional and structural characterization.

Escherichia coli↗

PubMed1996

Lipopolysaccharides and divalent cations are involved in the formation of an assembly-competent intermediate of outer-membrane protein PhoE of E.coli.

To identify the requirements for the biogenesis of outer-membrane proteins in Gram-negative bacteria, the sorting and assembly of the trimeric, pore-forming protein PhoE was studied in vitro. Purified lipopolysaccharide (LPS) in combination with low amounts of Triton X-100 and divalent cations induced the formation of folded monomers. LPS of deep-rough strains was far less efficient in the formation of folded monomers than wild-type LPS was. These folded monomers could be converted into heat-stable trimers upon addition of outer membranes and higher amounts of Triton X-100. Trimerization could precede the insertion step. These in vitro data suggest that the assembly in vivo proceeds sequentially by (i) formation of a folded monomer by interaction with LPS; (ii) sorting of the folded monomers to assembly sites in the outer membrane; (iii) trimerization; and (iv) insertion.

Cations, Divalent↗

PubMed1996

Porins of Escherichia coli: unidirectional gating by pressure.

OmpC and PhoE porins of Escherichia coli were examined by the patch-clamp technique following reconstitution in liposomes, and were observed primarily in the open (conducting) state. With application of negative voltage and positive hydrostatic pressure, OmpC exhibited marked gating towards a more closed state whereas PhoE remained largely unaffected by pressure application. Hybrid chimeric OmpC-PhoE proteins showed an increased tendency for pressure-dependent gating as the OmpC proportion in the chimeric molecule increased. In addition, several PhoE mutants with amino acid substitutions and insertions in either the L3 or L4 loop of the monomer exhibited pressure sensitivity comparable with the wild-type OmpC porin. Our data support the structural plasticity model of porins and are consistent with the 'charge-screening-unscreening' hypothesis that describes how these proteins may exist in distinct conformations.

Escherichia coli↗

PubMed1996

Escherichia coli outer membrane phospholipase A: role of two serines in enzymatic activity.

In the outer membrane phospholipase A (OMPLA) of Escherichia coli, Ser144 has previously been identified by chemical modification as the active site serine residue. In a specific OMPLA-negative mutant strain, the pldA gene coding for OMPLA was shown to differ from the wild-type gene by a single point mutation, resulting in the substitution of Ser152 by phenylalanine. The role in catalysis of these two serine residues in OMPLA was investigated by site-directed mutagenesis. Ser144 and Ser152 were replaced one at the time by either alanine, valine, phenylalanine, threonine, or cysteine. Ser152 was furthermore replaced by asparagine. Replacement of Ser144 by cysteine resulted in 1% residual activity, whereas the other substitutions at this position yielded virtually inactive enzymes. Substitution of Ser 152 by threonine or asparagine resulted in 40% and 2% residual activity respectively, whereas all other substitutions at this position resulted in the loss of enzymatic activity. We propose that Ser144 is the nucleophile in catalysis, and that Ser152 is involved in hydrogen bonding either to the catalytic triad or in the oxyanion hole.

Amino Acid Sequence↗

PubMed1996

prlA suppressors in Escherichia coli relieve the proton electrochemical gradient dependency of translocation of wild-type precursors.

The SecY protein of Escherichia coli is an integral membrane component of the protein export apparatus. Suppressor mutations in the secY gene (prlA alleles) have been isolated that restore the secretion of precursor proteins with defective signal sequences. These mutations have never been shown to affect the translocation of wild-type precursor proteins. Here, we report that prlA suppressor mutations relieve the proton-motive force (pmf) dependency of the translocation of wild-type precursors, both in vivo and in vitro. Furthermore, the proton-motive force dependency of the translocation of a precursor with a stably folded domain in the mature region was suppressed by prlA mutations in vitro. These data show that prlA mutations cause a general relaxation of the export apparatus rather than a specific change that results in bypassing of the recognition of the signal sequence. In addition, these results are indicative for a mechanism in which the proton-motive force stimulates translocation by altering the conformation of the translocon.

Adenosine Triphosphate↗

PubMed1996

In vitro insertion and assembly of outer membrane protein PhoE of Escherichia coli K-12 into the outer membrane. Role of Triton X-100.

The assembly of the in vitro synthesized outer membrane protein PhoE into purified outer membranes was investigated. The assembly appeared to be strongly stimulated by the presence of low amounts of Triton X-100 (optimal 0.08%, w/v). The role of Triton X-100 in the in vitro system was further examined. Pretreating outer membranes with Triton X-100 did not make the membranes competent for correct assembly, indicating that the detergent did not act on the membrane but at the protein level. PhoE became assembly-incompetent with a half-life of approximately 12 min and 90 s at 37 degrees C in the absence and presence, respectively, of 0.08% Triton X-100. Apparently, Triton X-100 induces an assembly-competent state in the PhoE protein with a very short half-life. Furthermore, the efficiency of correct assembly of PhoE was greatly reduced when outer membranes of deep rough lipopolysaccharide mutants were used, indicating an important role of lipopolysaccharides in the assembly of the porin.

Cell Membrane↗

PubMed1996

Characterization of type II protein secretion (xcp) genes in the plant growth-stimulating Pseudomonas putida, strain WCS358.

In Pseudomonas aeruginosa, the products of the xcp genes are required for the secretion of exoproteins across the outer membrane. Despite structural conservation of the Xcp components, secretion of exoproteins via the Xcp pathway is generally not found in heterologous organisms. To study the specificity of this protein secretion pathway, the xcp genes of another fluorescent pseudomonad, the plant growth-promoting Pseudomonas putida strain WCS358, were cloned and characterized. Nucleotide sequence analysis revealed the presence of at least five genes, i.e., xcpP, Q, R, S, and T, with homology to xcp genes of P. aeruginosa. Unlike the genetic organization in P. aeruginosa, where the xcp cluster consists of two divergently transcribed operons, the xcp genes in P. putida are all oriented in the same direction, and probably comprise a single operon. Upstream of xcpP in P. putida, an additional open reading frame, with no homolog in P. aeruginosa, was identified, which possibly encodes a lipoprotein. Mutational inactivation of xcp genes in P. putida did not affect secretion, indicating that no proteins are secreted via the Xcp system under the growth conditions tested, and that an alternative secretion system is operative. To obtain some insight into the secretory pathway involved, the amino acid sequence of the N-terminus of the major extracellular protein was determined. The protein could be identified as flagellin. Mutations in the xcpQ and R genes of P. aeruginosa could not be complemented by introduction of the corresponding xcp genes of P. putida. However, expression of a hybrid XcpR protein, composed of the N-terminal one-third of P. aeruginosa XcpR and the C-terminal two-thirds of P. putida XcpR, did restore protein secretion in a P. aeruginosa xcpR mutant.

Amino Acid Sequence↗

PubMed1996

Mode of insertion of the signal sequence of a bacterial precursor protein into phospholipid bilayers as revealed by cysteine-based site-directed spectroscopy.

The interactions between a bacterial precursor protein and phospholipids in bilayer-based model membrane systems is addressed in this study. The precursor-lipid interactions were assessed from the side of the lipid phase by fluorescence and electron spin resonance spectroscopy, using the precursor of the Escherichia coli outer membrane protein PhoE. The role of the signal sequence, as part of the precursor, in this interaction was investigated by using cysteine-based site-directed spectroscopy. For this purpose, purified cysteine-containing mutants of prePhoE, which were made by site-directed mutagenesis of the signal sequence part and of the mature part, and defined lipids were used. The location of the fluorescently labeled cysteine residues was established by resonance energy transfer and quenching experiments and those of the corresponding spin-labeled cysteine residues by paramagnetic relaxation enhancement. It was demonstrated that precursor-phospholipid interactions exist in model membrane systems and also that these interactions were dependent on the presence of anionic phospholipids and resulted in a deep insertion of (parts of) the precursor into the lipid bilayer. Furthermore, the results with the cysteine mutations in the signal sequence of the precursor indicate that both termini of the signal sequence are located near or at the membrane surface, with only the fluorescence of the labeled cysteines in the signal sequence part being protected against aqueous quenchers. The results demonstrate that, when part of the intact precursor, the signal sequence experiences similar lipid-protein interactions as do isolated signal peptides. They also indicate that the signal sequence inserts entirely as a looped structure into the membrane. In addition, the data also indicate that the mature part of the precursor has an affinity for the membrane.

Amino Acid Sequence↗

PubMed1996

Involvement of stress protein PspA (phage shock protein A) of Escherichia coli in maintenance of the protonmotive force under stress conditions.

The expression of specific PhoE mutant proteins leads to induction of the expression of the psp operon of Escherichia coli and the export of various plasmid-encoded precursors is retarded in a pspA mutant strain. Here, we have investigated the specific role of various Psp proteins in the export process. PspB and PspC are both inner membrane proteins that are involved in the regulation of the transcription of the psp operon. Precursor PhoE translocation was retarded in a pspB mutant strain to a similar extent as in a pspA mutant strain. The reduced translocation efficiencies in the various psp mutants could be complemented by expression of PspA from a plasmid, indicating that only PspA is required for efficient translocation. Mutant prePhoE proteins that can be translocated independently of the deltamu H+ appeared to translocate equally efficiently in a wild-type and in a pspA mutant strain. Furthermore, quantitative in vivo determination of the deltamu H+ showed that it specifically decreased in a pspA mutant strain upon expression of plasmid-encoded (mutant) prePhoE protein. Apparently, the translocation defects observed in a psp mutant strain are caused by a decrease of the delta mu H+ and PspA functions by maintaining the delta mu H+ under these conditions.

Bacterial Proteins↗